IMPROVED CATALYTIC COMBUSTION SYSTEM INCLUDING A SEPARATOR BODY

Abstract: A combustor for supporting the catalytic combustion of gaseous carbonaceous fuel contains a catalyst zone in which is disposed a catalyst body comprising at least one catalyst member, a separator zone comprising a separator body and a downstream zone where homogeneous combustion occurs. The catalyst member contains a carrier and a catalyst material deposited thereon. The catalyst body may, optionally, contain additional catalyst members downstream of the at least one catalyst member. The separator body contains a carrier-type monolith containing ceramic fibers in a matrix. One of the optional additional catalyst members may also contain such a monolith on which the catalyst composition is disposed.

Description: IMPROVED CATALYTIC COMBUSTION SYSTEM INCLUDING A SEPARATOR BODY BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an apparatus and process for the catalytically supported combustion of gas¬ eous carbonaceous materials, including natural gas and methane. In a more specific aspect, this invention re¬ lates to an apparatus and process for catalytically sup¬ ported combustion of natural gas or methane using a sup¬ ported palladium oxide catalyst. Description o Related Art Catalytically supported combustion processes have been described in the prior art, e.g., see U.S. Patent 3,928,961 to Pfefferle and U.S. Patents 4,065,917 and 4,019,316. The use of natural gas or methane in catalytic combustion has been taught in the art, as has the use of a palladium catalyst to promote such combustion oxidation. See U.S. Patent 3,056,646 to Cohn, wherein the use of pal¬ ladium catalyst to promote methane oxidation is disclosed, as is an operable temperature range of 271°C to 900°C (see column 2, lines 19-25). U.S. Patent 4,154,568 to Kendall et al, dated May 15, 1979 discloses a catalyst bed design comprising a plural¬ ity of carrier monoliths in the flow stream of the air/- fuel mixture, wherein the channel size in respective mono- liths decreases progressively for monoliths at progres¬ sively downstream positions, to provide substantially com¬ plete combustion in the catalyst bed (see column 1, lines 47-59) . SUMMARY OF THE INVENTION The present invention provides a combustor for cata¬ lytically promoting thermal comoustion of an inlet combus¬ tion gas mixture flowed sequentially through an upstream zone and then a downstream zone of the combustor. The combustor comprises a catalyst body disposed in the up¬ stream zone and comprising at least one catalyst member comprising a carrier having a plurality of gas flow chan- nels extending therethrough defined by channel walls. The channel walls of the carrier have disposed thereon a cata¬ lyst composition effective for promoting combustion of the combustion gas mixture. There is also a separator body, disposed in the upstream zone in a position downstream of the catalyst body. A homogeneous reaction zone is dis¬ posed within the downstream zone. The separator body is dimensioned and configured to thermally shield the cata¬ lyst body from the homogeneous reaction zone. According to one aspect of the invention, the cata- lyst body and the separator body may together comprise a single, monolithic body. Another aspect of the invention provides that the catalyst body and the separator body may comprise discrete bodies disposed in proximity to each other or in mutual abutting relation to one another. The catalyst body and the separator body may be of substantially identical cross-sectional area and configuration. Yet another aspect of the invention provides that the catalyst composition of the catalyst body may comprise a palladium oxide catalyst composition. The palladium oxide catalyst composition may comprise palladium oxide dispers¬ ed on a refractory inorganic oxide support, such as alu¬ mina or alumina impregnated with a rare earth oxide. The catalyst body many comprise a plurality of cata- lyst members, and the respective catalyst compositions thereon may be different from, or the same as, one an¬ other. On the other hand, the separator body is prefer¬ ably, but not necessarily, substantially free of a metal- containing catalyst, e.g., a palladium-containing cata- lyst, effective for catalytically promoting thermal com¬ bustion of the inlet combustion gas mixture. Thus, the separator body, if it has a coating thereon, may be coated with alumina. The coating on the separator body may com- prise a palladium containing material that is substantial¬ ly inactive for the combustion of the combustion gas mix¬ ture under normal separator body operating conditions, e.g., the coating may comprise a binary oxide of a rare earth metal, e.g., lanthanum and palladium. In preferred embodiments of the present invention, the catalyst body may comprise a plurality of catalyst members. The catalyst members may be disposed in abutting contact, or in proximal relation to one another. The cat- alyst members and the separator body may have from about 9 to about 400 gas flow channels per cros's-sectional square inch ("cpsi"). The total combined length of the catalyst members may be from about 1/2 to 12 inches, and the length of the separator body may be from about 1/2 to 5 inches. According to still another aspect of the present in¬ vention, the separator body may comprise a silica-magne- sia-alumina material comprised primarily of cordierite, mullite and corundum. The silica-magnesia-alumina materi¬ al may comprise from about 20 to 40 weight percent Si0 2 , from about 3 to 6 weight percent MgO and from about 54 to 77 weight percent Al.,0... About 50 to 90 percent by weight of each of said Si0 2 , MgO and Al 2 0 3 may comprise crystal¬ line material, the balance comprising amorphous material. The crystalline material typically comprises about 15 to 40 percent by weight cordierite, about 15 to 35 percent by weight corundum and about 10 to 30 percent by weight mul¬ lite, based on the weight of the carrier. Alternatively, the separator body may comprise a ceramic fiber matrix material comprising ceramic fibers, the composition of which comprises alumina, boron oxide and silica, the fi¬ bers being fixed in a silicon carbide matrix. The fibers of the ceramic fiber matrix material may comprise, for example, about 64 percent alumina, 14 percent B_0, and 24 percent SiO, . As used herein and in the claims, the terms "up¬ stream" and "downstream" refer to the relative placement of elements sensed in the αirection of flow of zne combus¬ tion mixture through a catalyst apparatus according to the invention . BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic plan view of a gas turbine unit utilizing catalytic thermal combustors in accordance with one aspect of the present invention; Figure 2 is a schematic longitudinal cross-sectional view of one of the catalytic thermal combustors of Figure 1 showing four cylindrical catalyst members arranged therein; Figure 2A is' a view taken along line A-A of Figure 2 showing a cross section of catalyst member 1 of Figure 2; Figure 2B is a view, greatly enlarged with respect to Figure 2A, showing in cross section one of the gas flow channels of catalyst member 1; Figures 3A and 3B are SEM photographs of cross sec¬ tions taken from the inlet and outlet ends, respectively, of segment SI of Example 1; Figures 3C and 3D are SEM photographs of cross sec- tions taken from the inlet and outlet ends, respectively, of spent segment S2 of Example 1; Figures 3E and 3F are SEM photographs of cross sec¬ tions taken from the inlet and outlet ends, respectively, of segment S3 of Example 1 after aging; Figures 3G and 3H are SEM photogrphas of cross sec¬ tions taken from segment S4 of Example 1 after aging; Figures 4A and 4B are SEM photographs of cross sec¬ tions of inlet and outlet ends, respectively, of segment C3 of Example 2; Figures 4C and 4D are SEM photographs of cross sec¬ tions of spent and fresh spent samples, respectively, of segment C4 of Example 2; Figures 5A and 5B are SEM photographs of cross sec¬ tions of spent segments E3 and E4 of Example 2, respectively; Figures 6A and 6B are SEM photographs of cross sec¬ tions taken from the inlet and outlet ends, respectively, of spent segment F3 of Example 2; Figures 7A and 7B are SEM photographs of cross sec¬ tions taken from the inlet and outlet ends, respectively, of segment HI of Example 3; Figures 7C and 7D are SEM photographs of cross sec- tions of spent segment H2 of Example 3; Figures 7E and 7F are SEM photographs of cross sec¬ tions taken from the inlet and outlet ends, respectively, of segment H3 of Example 3; and Figures 7G and 7H are SEM photographs of cross sec- tions taken from the inlet and outlet ends, respectively, of segment H4 of Example 3. DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS THEREOF Burning of carbonaceous fuels is associated with for¬ mation of air pollutants, among the most troublesome of which are nitrogen oxides (NO ). Nitrogen oxides form whenever air-supported combustion takes place at open- flame temperatures. One approach to eliminating nitrogen oxides involves catalytic post-treatment to reduce NO χ to nitrogen. A more economical method is to operate the com¬ bustion process catalytically, at a temperature lower than open-flame temperatures. It has long been realized that little or no NO χ is formed in such a system. Typically, such catalytic com¬ bustion of natural gas or methane, for example, utilizes a preburner or thermal combustor which employs flame combus¬ tion to preheat combustion air to a temperature of 350°C or higher. Once the catalyst is sufficiently hot to sus¬ tain catalysis, the preburner is shut down and all the fuel and air are directed to the catalyst. Such a cata¬ lytic combustor, if operated at temperatures below about 1300°C to 1500°C, avoids or at least controls to accept- able levels the NO χ formation which occurs at the higher temperatures which are characteristic of the flame combus¬ tion. However, suc catalytic combustion which will func¬ tion effectively at a high space velocity has heretofore been generally regarded as commerically unattractive. Reasons for this lack of commercial attractiveness include the difficulty of economically combusting methane, the principal component of natural gas, and the deactivation and instability of the catalyst compositions employed, especially in the high-temperature end of the catalyst bed where severe high temperatures may be reached. Because of the susceptibility of the catalyst to such thermal deacti¬ vation, many catalytic combustor designs are limited with respect to the type and amount of fuel they can combust in order to avoid deleterious high temperatures. The present invention serves, in a broad aspect, to alleviate catalyst failures by providing a thermal buffer or separator body disposed between the catalyst body and a downstream zone where high temperature homogeneous combus¬ tion occurs. The separator body comprises a monolith which is preferably similar in configuration to the carri¬ er substrates described below for catalyst materials, i.e., it may take the form of a honeycomb monolith having a plurality of parallel gas flow passages extending there¬ through. The separator body may be made of any material that can withstand exposure to the high temperatures pro¬ duced by the homogeneous combustion that occurs in the neighboring downstream zone of the combustor. Due to its placement between the catalyst zone and the downstream zone where homogeneous combustion occurs, the separator body at least partially insulates the cata¬ lyst body from the heat released by the homogeneous com¬ bustion reaction. Preferably, but not necessarily, the separator body does not comprise catalytically active ma¬ terials, since such materials tend to be vunerable to deactivation when exposed to the temperatures that the separator body may sometimes experience. Further, cata¬ lytic material may accelerate the combustion reaction in the separator zone, thus generating additional heat at a point so near the catalyst zone as to exacerbate the risk of exposing the cacalyst materials in the catalyst zone to excessive temperatures. Therefore, the use of non-cata- lytic separator body reduces the likelihood of thermal de¬ activation of the catalyst body in the catalyst zone. The separator body is disposed on the downstream side of the catalyst body, either in abutting relation thereto or in close proximity thereto sufficiently close so that the channeled flow of gases through the catalyst body is sub¬ stantially preserved as channeled flow through the separa¬ tor body. One type of separator body able to withstand the high temperatures that prevail in the nearby homogeneous com¬ bustion zone is available in the form of a honeycomb-type monolith from the Minnesota Mining and Manufacturing Co. (3M) under the trade designation "Siconex." These mono¬ lith substrates are described by the manufacturer as being formed from a series of layers of woven alumina-boria-sil- ica inorganic fibers. The thus formed monolith is then coated with silicon carbide in a vapor deposition process which is believed to enclose the fibers in a silicon car¬ bide matrix. A surface layer of silica is believed to form on the silicon carbide matrix when the monolith is calcined. Surprisingly, these monoliths have been found to exhibit better long-term thermal strength than more conventional monoliths. The 3M Company provided an assay of its Siconex monolith, which describes the monolith as comprising about 70% silicon carbide and about 30% NEXTEL™ 312 ceramic fibers. The NEXTEL™ 312 ceramic fibers are described as comprising an alumina-boria-silica material comprising 62 weight percent Al 2 0 3 , 14 weight percent B 2 0 3 and 24 weight percent Si0 2 . As will be discussed below, Siconex-type substrates can sometimes be used as carriers for catalyst materials to provide a catalyst member in a catalyst zone, and in such use are referred to herein as Type II carriers to distinguish from more conventional carriers referred to as Type I carriers, as described be- low. Optionally, a honeycomb monolith-type separator body may be coated with a coating comprising a refractory inor¬ ganic oxide, e.g., alumina. Other refractory oxides may be used as well and are known in the art, such as zirco- nia, titania, ceria, etc., and mixtures thereof. Like the catalyst member, which may, as discussed below, comprise one or more carrier monoliths, the separator body may also comprise one or more monoliths each of which may have a catalytically inert coating thereon. In other embodi¬ ments, the coating on the separator body monolith may comprise catalytically active species. ithin the catalyst zone upstream of the separator body is disposed a catalyst body comprising at least a first catalyst member comprising a carrier coated with a catalyst material. Generally, the catalyst material com¬ prises a catalytically active metal or metal oxide, typi¬ cally a platinum group metal or metal oxide such as palla- dium oxide, dispersed on a refractory metal oxide support material such as alumina. The choice of catalyst material may be guided by the type of fuel being combusted. As suggested above, natural gas is a common fuel, but the present invention finds utility in processes for combust- ing other fuels as well, e.g., number 2 fuel oil, jet fuel, normally liquid hydrocarbon fuels, alcohols, e.g., methanol, oxygenated hydrocarbons, and even hydrogen, which may be reacted with carbon monoxide. In addition, the expected operating conditions of the combustor may also be taken into account in choosing catalysts. Cata¬ lyst materials may be formed as a slurry and thus deposit¬ ed onto the carrier monoliths in a process well known in the art. As will be discussed below, the catalyst zone may comprise a plurality of catalyst members, each of which may comprise the same or different catalytic materi¬ als. The catalyst members of the catalyst body are adapt¬ ed to initiate in the catalyst zone catalytically-support- ed, i.e., heterogeneous, combustion at the surfaces there¬ of and to support thermal flame, i.e., homogeneous, tem- perature combustion in the downstream zone. Typical catalyst materials for the combustion of na¬ tural gas include palladium oxide as the active component, with the palladium oxide dispersed on a support material comprising a relatively inert refractory inorganic oxide such as alumina, which is optionally impregnated with sta¬ bilizers, promoters or other additives. Other support materials such as silica, titania, unimpregnated zirconia, zirconia impregnated with a rare earth metal oxide, ceria, co-formed rare earth metal oxide-zirconia and combinations thereof may also be employed. The palladium oxide is dis¬ persed on the support material in a conventional manner, e.g., by impregnating particles of the support material with a solution of a soluble palladium compound and then calcining the impregnated material. The support materials may be stabilized against thermal degradation, e.g., by the impregnation of stabilizing species, to provide a ca¬ talyst material better suited for use at a relatively downstream position in the catalyst zone. Further still, alternative active components may be employed, such as bi¬ nary oxides of palladium and rare earth metals as disclos¬ ed in co-pending, commonly assigned U.S. Patent Applica¬ tion Serial No. 07/684,409, filed April 12, 1991 and co- pending, commonly assigned U.S. Patent Application Serial No. 07/684,631, filed April 12, 1991, the disclosures of which are hereby incorporated herein by reference. These binary oxides may result from the solid state reaction of palladium oxide with the rare earth metal oxides, to pro- duce, e.g., Sm. PdO, , Nd 4 Pd0 7 , Pr 4 Pd0 7 or La 4 Pd0 7 . Such alternative active components are typically admixed with a refractory metal oxide binder to bind the material to the carrier. Other catalyst materials known in the art may be used as well. The carrier on which the catalyst composition is car¬ ried is typically a monolith having a plurality of fine gas flow passages extending therethrough, to provide a honeycomb-type structure. The gas flow passages (some¬ times referred to as "cells") in the honeycomb structure are substantially parallel and defined by thin walls, and may be of any desired cross section such as square, rect¬ angular, triangular or hexagonal shape. The number of channels per square inch of face surface, i.e., per cross-sectional square inch (cpsi), may vary, depending upon the particular application for which the catalyst bed is to be used. Such honeycomb-type carriers are commer¬ cially available having anywhere from about 9 to 600 or more cpsi. The substrate or carrier monolith desirably is porous and may (but need not) be relatively catalytically inert to the combustion reaction as compared to the active layers used in the invention. The carrier used in a catalyst body of the present invention should be refractory in nature, i.e., able to withstand thermal shock caused by the sudden increase or decrease in temperature experienced at start-up and shut¬ down of the combustor. The carrier should also have good thermal strength so that it does not develop structural flaws at the operating temperatures of the combustor, i.e., temperatures as high as 1500°C. Conventional cor¬ dierite monoliths such as those used to support three-way catalysts for treating the exhaust gases of automotive in¬ ternal combustion engines are generally not considered to be suitable in combustors of the present invention because they can melt or otherwise fail at combustor operating temperatures. Suitable carriers may comprise a combina¬ tion of cordierite and other oxide materials, e.g., a mix¬ ture of alumina, mullite and cordierite. Such carriers have physical properties more suited to combustor opera¬ tion than conventional ceramic substrates, typically used to carry catalysts used in the treatment of automotive exhaust gases, i.e., they exhibit better thermal strength and thermal shock resistance, and are commercially avail- able, e.g., from the Dupont Company under the designation PRD-66. An elemental analysis of this material provided by the Dupont Company describes the material containing 70.4 weight percent Al 2 0 3 , 24.9 weight percent Si0 2 and 4.2 weight percent MgO. However, another analysis re- suited in proportions of about 62.7 - 63.4 weight percent A1,0 3 , 31.2 - 31.3 weight percent Si0 2 and 5.4 - 5.7 weight percent MgO. Approximately 5C to 90 percent by weight of each of the Si0 2 , MgO and Al 2 0 3 may comprise crystalline material, the balance comprising amorphous ma¬ terial. Typically, the crystalline material comprises 15 to 40 percent cordierite, 15 to 35 percent corundum and 10 to 30 percent mullite by weight of the carrier. A further description of this material may be found in U.S. Patent 5,079,064, the disclosure of which is hereby incorporated herein by reference. Carriers comprising such materials are sometimes referred to herein as Type I carriers. As indicated above, the catalyst body may comprise more than one catalyst member. In one perferred embodi¬ ment, it may be desired to emplace catalyst members having different catalyst materials in the catalyst zone in a se¬ quence suited to the temperature conditions of operation of the combustor. Co-pending, commonly assigned patent application Serial No. filed on teaches how catalyst members may be sequenced according to temperature-related characteristics of catalyst materials disposed thereon, and the disclosure of that application is hereby incorporated herein by reference. Briefly re- stated, the cited patent application teaches that catalyst materials should be disposed in relative upstream-down- stream relation in order of at least one of decreasing catalytic activity, increasing thermal stability (i.e., escalating degradation temperature) or escalating and preferably overlapping regeneration temperature ranges. In another preferred embodiment, the catalyst body comprises two catalyst members, both comprising a catalyst material comprising PdO dispersed on a support material comprising alumina impregnated with rare earth oxide, e.g., ceria. The carriers for these catalyst members are Type I carriers. Preferably, the separator body comprises two Type II monoliths. Preferably, the first catalyst member, each optional additional catalyst member and the separator body are dis- crete bodies within the combustor. For example, the first catalyst member will preferably comprise the first cata¬ lyst composition disposed on the first carrier and the second catalyst member will likewise comprise the second catalyst composition on a separate second carrier. Then, the first catalyst member and the second catalyst member may be disposed within the combustor in adjacent, option¬ ally abutting, upstream/downstream relation to one anoth- er. The catalyst members may be disposed with their re¬ spective gas flow channels in mutual alignment so that the flow of combustion gases through the first catalyst member will be channeled into the second catalyst member. The first catalyst member and the second catalyst member may be formed on a single, integral monolith by applying a coating of the first catalyst composition on one end of the monolith and a coating of the second catalyst composi¬ tion on the other end of the monolith. The separator body preferably also comprises a discrete refractory body having a plurality of gas flow channels extending there¬ through and has a cross-sectional area and configuration substantially identical to that of the adjacent catalyst member and is aligned in this manner as well. In other embodiments of the invention, the separator body and a downstream catalyst body may together comprise a single monolith, e.g., a Type II monolith, one end of which is coated with a catalyst material to form the catalyst body, the other end of which is preferably either uncoated or is coated with a relatively inert, i.e., non-platinum group metal-bearing, coating. Referring now to Figure 1 there is shown in schematic plan view a gas turbine 10 comprising a starter engine 12 connected by an engine shaft 14 to an air compressor 16, which is provided with inlet air, via air inlet lines in- dicated by arrows a, which is compressed by compressor 16 and discharged via lines a' into combustion gas inlet lines c which are also supplied with a pressurized gaseous fuel, such as natural gas or methane, via gas inlet lines indicated by arrows f. The air and fuel combine to form a combustion mixture which is introduced via lines c into a plurality of catalytic thermal combustors 18, two of which are illustrated in Figure 1 although it will be appreci¬ ated that any suitable number may be employed. For exam- ple, eight such combustors 18 may be utilized with their outlets disposed equiradially about the inlet to the tur¬ bine. Each catalytic thermal combustor 18 is provided with an associated outlet duct 20 connected in gas flow communication with a turbine 22 which may comprise a mul¬ ti-staged turbine as well known to those skilled in the art. Turbine 22 is drivingly connected to a load coupling shaft 24 to connect the turbine output to a suitable de¬ vice, for example, an electric generator. The expended combustion products are exhausted as shown by arrow e via exhaust stack 26 for discharge to the atmosphere or for further use or processing. Figure 2 shows a schematic cross-sectional view of a typical catalytic thermal combustor 18 comprising a canni- ster 19 having an inlet section 28, an upstream zone 30 wherein is disposed a catalyst body comprising catalyst members 1, 2, and 3 and a separator body 4, and a down¬ stream zone 32. The three catalyst members 1, 2, and 3, and separator body 4 are arranged in abutting contact. That is, catalyst members 1 and 2 are positioned in face- to-face abutting contact, as are catalyst members 2 and 3. Separator body 4 is in abutting contact with catalyst mem¬ ber 3. Generally, the catalyst members 1, 2, and 3 each comprise a refractory honeycomb monolith carrier. The carrier is a substantially cylindrical body (see Figure 2A) having opposite end faces between which extend a plurality of generally parallel, fine gas flow passages. Figure 2A shows a typical catalyst member end face la of catalyst member 1, schematically showing a plurality of fine, parallel gas flow passages extending longitudinally through catalyst member 1 to permit gas flow through cat¬ alyst member 1. This construction is typical of all the catalyst members 1 through 3 inclusively. The gas flow passages are defined by walls on which are disposed a coating (often referred to as a "washcoat") of an active material suitable to catalyze the oxidation of a gaseous fuel such as natural gas or methane. Figure 2B shows an enlarged view corresponding to Figure 2A in which a typical gas flow passage 34 in a cat¬ alyst member is shown in cross-sectional view as being de¬ fined by four gas flow passage walls 34a on which is coated a catalytic material washcoat 36. The cross-sec- tional configuration of gas flow passage 34 illustrated in Figure 2B is rectangular but it will be appreciated that any suitable cross-sectional configuration may be employed such as square, polygonal, e.g., triangular, or circular. Further, the gas flow passages may have a configuration attained by alternating layers of flat and wave-form plates made of a suitable refractory material, as is well known to those skilled in the art. Preferably, separator body 4 is dimensioned and con¬ figured to provide gas flow channels that correspond with the channels in catalyst member 3, i.e., the catalyst mem¬ ber against which the separator body is disposed. This allows the gas stream to maintain channeled gas flow from the catalyst member through the separator body. Example 1 To demonstrate the effectiveness of using a separator body according to the present invention to thermally in- sulate catalytically active segments upstream thereof. A catalyst bed was prepared which comprised four segments, all of which comprised Type I monoliths having 64 cells per square inch. Segments 1,3 and 4 were each 1.5 inches long and segment 2 was 1 inch long. Segments 1 and 2 were coated with catalyst material, which, on segment 1, comprised 4% by weight of the cata¬ lyst material palladium as palladium oxide dispersed on an alumina support. This material was prepared in a conven¬ tional manner, i.e., by impregnating activated alumina support material with a palladium salt solution and drying and calcining the impregnated alumina. Segment 2 carried a catalyst material comprising 8% by weight palladium as palladium oxide and 10% by weight cerium oxide on an alu- mina support. The palladium and cerium oxide were co- impregnated into the alumina by preparing a solution of cerium nitrate and palladium nitrate, impregnating the alumina with the solution, and then drying and calcining the co-impregnated alumina. Segments 3 and 4, which pro¬ vide the separator body, each were coated with alumina. The washcoat loading on each segment was 1.5 grams per cubic inch. The configuration of this catalyst bed, which is designated bed S, is set forth in the Table IA below. The segments were placed as a catalyst bed in a com¬ bustor and was used to catalytically support the combus¬ tion of a combustion mixture comprising 4% methane in air flowing at a speed of 50 feet per second at a pressure of 3 atmospheres. Combustion was initiated and terminated 70 times and the bed ran for a total of 500 hours, igniting combustion at inlet temperatures in the range of about 480° - 520°C. The gases exiting from the combustor gener¬ ally contained less than 1.5 parts per million of nitrogen oxides and about 4 - 15 parts per million carbon monoxide. An analysis unit detected no unburned hydrocarbons. The length of the run established that the catalyst bed had sufficient durability for catalytically initiating com- bustion of the air/fuel mixture. Following the 500 hour trial, the segments were sep¬ arately evaluated for catalytic activity, and their re- spective structural integrities were examined by means of scanning electron microscopy and an energy dispersion spectroscopy (SEM/EDS). In addition, similar samples were prepared for comparison, using fresh materials. The activity testing was performed by taking a sample core from each segment and flowing a mixture of 1% methane in air at a rate of 20 feet per second through the core sample and raising the inlet temperature of the methane/ air mixture. Temperatures at which specific percentages of conversion were achieved were noted. Samples were taken from both the inlet and outlet ends of these seg¬ ments. The results are set forth below in TABLE IB. Segment SI fresh spent inlet spent outlet 52 fresh spent inlet spent outlet 53 fresh spent inlet spent outlet The data of TABLE IB show that segments SI and S2 suffered only minor deactivation following the 500 hour combustion run due to shielding by separator body segments S3 and S4. The apparent increase in activity of segment S3 may be attributed to palladium deposits that migrated onto segment S3 from segments SI and S2. The SEM/EDS examination of segment SI revealed that the segment maintained structural integrity, showing lit¬ tle interaction between the washcoat and the substrate at both the inlet and outlet ends, with little washcoat loss. Segment S2 also showed little washcoat loss, but it ap¬ peared that the washcoat suffered more degradation than that of segment SI. In addition, some loss in structural integrity was evident in segment S2. SEM photos of cross- sectional segments SI, S2, S3 and S4 are shown in Figures 3A - 3H. The photos of segment S4 show that this segment suffered greater deterioration than any other segment of the bed. These observations, taken in light of the dura¬ tion of combustor operation and in conjunction with the activity data presented above, show that the separator body of the present invention is effective to thermally shield the upstream catalyst members from the deleterious high temperatures produced in the homogeneous combustion zone. Example 2 To illustrate the use of separator bodies according to the present invention having different compositions and to demonstrate the superior resistance to thermal degrada¬ tion of catalyst members and separator bodies comprising Type II substrate, four additional catalyst beds desig¬ nated bed C, bed D, bed E and bed F, each comprising four catalyst members, were prepared. TABLE IIA summarizes the respective configurations of the four catalyst beds. The catalyst materials for segments 1 and 2 in each bed in this Example were prepared in the manner described in the above Example 1. The catalyst material on members C3 and F3 comprised 7% La 4 Pd0 7 and 93% alumina as a binder. The La 4 PdO-. was prepared by mixing La,0, with palladium oxide in selected weight ratios. The mixture was mechanically ground to a size range of about 50 to 100 micron diameter particles. The grinding was followed by calcination in air, for example, at a temperature of about 1100°C for about 66 hours to provide a reaction mixture containing the binary oxide of palladium and lanthanum. Preferably, the lanthana and palladium oxide starting materials are mixed in stoichiometric proportions to produce the desired compound. Thus, the molar ratio of the lanthana to PdO in the reaction mixture may be 2:1, 1:1 or 1:2. Although it is not necessary to use the starting materials in the mo¬ lar ratios of the desired binary oxide product, the use of such stoichiometric proportions has been found to be ad¬ vantageous, as described in aforesaid U.S. patent applic¬ ation Serial No. 07/684,409. All the Type I substrates in catalyst beds C, D, E and F had 64 cells per square inch, and all the Type II substrates had 60 cells per square inch. The washcoat loadings on the catalyst members of beds C, D, E and F was 1.5g/in ' m all cases. The configurations of beds C, D, E and F are summarized in TABLE IIA. C3 1.5' 7 wt.% 2La.0 3 .PdO/ 93% alumina C4 1.5" alumina Catalyst Bed D shcoat TABLE IIA (CONTINUED) Catalyst Bed E Catalyst Substrate Member Type, Length Washcoat El Same as Cl Same as Cl E2 Same as C2 Same as C2 E3 II 1.5" alumina E4 II 1.5" alumina Catalyst Bed F Catalyst Substrate Member Type, Length Washcoat Fl Same as Cl Same as Cl F2 Same as C2 Same as C2 F3 II 1.5" 7 wt.% 2La 2 0 3 .PdO/93% alumina F4 II 1.5' alumina The efficacy of catalyst beds C, D, E and F were tested by placing them in a combustor to determine their respective initiation temperatures for a 4% methane in air combustion mixture at 3 atmospheres pressure. Two evalua¬ tions were performed for beds C and E, and three evalua¬ tions were made for catalyst beds D and F. The results are set forth below in TABLE IIB. TABLE IIB Ignition Conditions Cat. Inlet Init. Vel. Fuel Cone. Extinction Bed Tem . (oC) (ft/s) Temp. (oC)/Fuel % 480-500 50 4.0 462-480 / 4.0 420 30 4.0 D 487-550 50 4.1-3.75 530 50 4.0 496 4.0 485-495 30 4.0 465-485 / 4.0 TABLE IIB CONTINUED ) The data of TABLE IIB show that catalyst beds E and F, which comprise Type II monoliths, provide catalytic activity comparable to that of catalyst beds C and D, which comprise only Type I monoliths throughout. The foregoing catalyst beds C, D, E and F were aged by placing them in a combustor and passing a combustion mixture comprising 4% methane in air at an inlet linear velocity of 30 to 60 feet per second to initiate combus¬ tion for a period of 4 to 20 hours at 3 atmospheres pres¬ sure. Thereafter, samples of the spent catalyst members were examined by scanning electron microscope and compared against fresh (unaged) samples for visual evidence of de- terioration. In some cases, samples were taken from both the inlet end and the outlet end of a particular catalyst member. Figure 4A and Figure 4B are SEM photographs of a cross section of spent catalyst member C3 taken at the in- let and outlet ends, respectively, and clearly reveal that the outlet end of catalyst member C3 suffered greater de¬ terioration than the inlet end. Energy Dispersion Spec- troscopy ("EDS") showed a loss of palladium on the cata¬ lyst material of catalyst member C3. Figure 4C is a SEM photograph of a cross section of the aged separator body C4 showing evidence of deterioration and washcoat-sub- strate interaction with the Type I substrate therein. Figure 4D is a SEM photo of a cross section of an unused separator body of the same composition as separator body C4. Figures 4A-4D demonstrate that Type I substrates dis¬ posed in the downstream portion of the catalyst bed inter¬ act under operating conditions with the alumina-containing layer thereon, with a tendency toward greater interaction at more downstream positions. Figures 5A and 5B are SEM photographs of cross sec¬ tions of separator bodies E3 and E4 showing little deteri¬ oration and alumina-containing coating material-substrate interaction. Figures 6A and 6B are SEM photographs of cross sec¬ tions of the inlet and outlet ends of catalyst member F3 indicating that the structural integrity of segment F3 was not materially affected at either end. EDS analysis show- ed no significant loss of palladium from the washcoat at either end of this segment. <' The foregoing description of beds E and F show not only that Type II monoliths function well as separator bodies, but that, surprisingly, Type II substrates exhibit better resistance to structural deterioration then Type I substrates. Therefore, there is a reduced chance that a separator body comprising a Type II substrate will fail physically under the stresses of combustor operation. It is believed that the catalyst material on members C and F3 degraded during their respective combustor runs, and thus became catalytically inactive and were thus con¬ verted into separator bodies. Although the coatings on separator bodies generally do not include catalytic me¬ tals, e.g., platinum group metals, the deactivated 2 La,0, .PdO material provides a potential catalytic material that may be activated by exposure to conditions that allow the material to regenerate into a catalytically active compound. Further, it was apparent that member F3 retain¬ ed more palladium than did member C3. Therefore, in the event that the upstream catalyst members failed and the catalyst beds cool sufficiently to allow the catalyst ma¬ terial on members C3 or F3 to regenerate, it is likely that bed F would show better performance after regenera¬ tion than bed C, due to the greater quantity of palladium retained on segment F3. Example 3 A catalyst bed H having two catalyst members was pre¬ pared utilizing Type I substrates in the 1st and 2nd posi- tions and two separator bodies comprising Type II sub¬ strates in the 3rd and 4th positions. Catalyst member HI had a catalyst material thereon comprising 8 weight per¬ cent palladium on alumina and was 1 inch in length. The catalyst material on catalyst member H2 comprised 4 weight percent palladium on alumina impregnated with 10 percent ceria, and had a length of 1.5 inches. Separator bodies H3 and H4 were both coated with lanthana- and baria- impregnated alumina and were 1.5 inches in length. The La-Ba alumina was prepared by impregnating the alumina with solutions of barium and lanthanum salts, and then drying and calcining the impregnated alumina to provide a support material comprising about 1.35% baria and about 1.85% lanthana by weight of the material. Other rare earth metals accompanied the lanthanum compound used to prepare this material so that the total rare earth oxides in the finished material comprised about 95% by weight lanthana and about 5% of oxides of other rare earth me¬ tals, typically ceria and neodymia. The Type I substrate had 64 cells per square inch; the Type II substrates had 60 triangular cells per square inch. The configuration of bed H is summarized in TABLE IIIA below. TABLE IIIA When fresh, catalyst members Hi and H2 had a brown color; separator bodies H3 and H4 were white. Before bed H was assembled, segments Hi and H2 were placed in a preliminary combustor bed in which segments 3 and 4 were separator bodies having a coating of alumina thereon. The preliminary bed experienced 10 ignitions and was subjected to 12.75 hours of on-stream time, yielding 6.25 hours of complete combustion. Separately, segments H3 and H4 were placed in the downstream positions of a different preliminary bed which experienced 5 ignitions in 2.75 hours of on-stream time but no complete combustion. Subsequently, segments H3 and H4 were used in still an¬ other preliminary bed which experienced 6 ignitions in 5.5 hours of on-stream time, yielding 1.75 hours of complete combustion. Following the preliminary combustion runs, bed H was assembled in a combustion test unit which was operated at 1 atmosphere pressure. The approach velocity of the gas stream was 17 m/sec (56 ft/sec), and the combustion mix- ture was about 4 to about 5.2 volume percent methane in air. The bed experienced 1 ignition, 3.2 hours of on- stream time and 2.0 hours of complete combustion. Combus¬ tion was ignited at 537°C at a combustion mixture methane content of 4.2 volume percent. Following ignition, the inlet temperature and volume percent fuel were varied to determine the performance of the bed under various condi¬ tions. The inlet velocity and pressure were steadily maintained. The performance data, including temperature measurements made downstream of segment HI, segment H2 and at a point 6 inches downstream of the catalyst bed, are summarized in TABLE IIIB. TABLE 11 IB Effluent Components Outlet NO CO UHC ] (ppm) (ppm) (ppm) (V%) UHC stands for unburned hydrocarbons * The values given for Hi and H2 exit temperature are the average of two measurements taken at the exit of each piece. The attainment of ignition and complete combustion and the data of TABLE IIIB show that segments HI and H2 were effective in bed H to catalyze the combustion of the air-fuel mixture despite their prior combustion runs, and thus demonstrate that the separator bodies used downstream of segments HI and H2 in those prior runs effectively pro¬ tected these segments from exposure to deactivating tem¬ peratures. 0790 -25- Catalyst Activity Two test cores measuring 0.75 inches in diameter and 0.5 inches in length were taken from the inlet and outlet ends of spent catalyst members Hi and H2. A similar test core was taken from a fresh catalyst member of each type. The activity of the samples was measured in the manner described above in Example 2 and the results are set forth in TABLE IIIC below. TABLE IIIC Catalyst Temp. ( C) at %CH Conversion Member Tθ% 20% 30% 40% 50? Segment HI (8% Pd/A1 : Q. ) fresh 340 370 403 463 592 700(55%) spent inlet (i) 405 459 508 559 625 700(55%) spent inlet (ii 422 495 552 603 670 700(54%) spent outlet( i ) 410 460 511 564 630 700(55%) spent outlet(ii 431 498 555 607 678 700(53%) Segment H2 (4%Pd,10%CeO. /Al.0- fresh 390 451 517 598 700(44% spent inlet (ι) 420 501 550 652 700(40% spent inlet (ii) 454 526 594 683 700(41% spent outlet(i) 471 543 601 687 700(41% spent outlet(ii) 480 550 613 690 700(41% The data of TABLE IIIC show that the inlet and outlet portions of catalyst member HI show roughly the same de- gree of deactivation after a combustion cycle. The outlet portion of catalyst member H2 was more greatly deactivated than the inlet portion. Structural Integrity SEM photographs of cross-sections taken from the in¬ let and outlet ends of spent catalyst members Hi and H2 are shown in Figures 7A and 7B (Segment Hi) and 7C and 7D (Segment H2) , respectively. These photos show very little deterioration in catalyst members HI and H2 indicating the efficacy of separator bodies H3 and H4, which also showed good washcoat retention and good substrate integrity, as seen in Figures 7E, 7F and 7G, 7H. The post run activity data of TABLE IIIC and the foregoing structural integrity study show that the use of separator bodies H3 and H4 effectively shielded catalyst members Hi and H2 upstream thereof from thermal degrada¬ tion. The structural integrity results also show that separator bodies comprising Type II monoliths resist de¬ trimental interaction with a washcoat thereon, thus better preserving their mechanical strength. While the invention has been described with reference to particular embodiments thereof, it will be appreciated that numerous variations to the described embodiments will be within the scope of the appended claims.

Claims: THE CLAIMS What is claimed is: 1. A combustor for catalytically promoting thermal combustion of an inlet combustion gas mixture flowed se¬ quentially through an upstream zone and then a downstream zone of the combustor, the combustor comprising: a cata¬ lyst body disposed in the upstream zone and comprising at least one catalyst member comprising a carrier having a plurality of gas flow channels extending therethrough defined by channel walls, the channel walls of the carrier having disposed thereon a catalyst composition effective for promoting combustion of the combustion gas mixture; a separator body disposed in the upstream zone in a position downstream of the catalyst body; and a homogeneous reac¬ tion zone disposed within the downstream zone; the separator body being dimensioned and configured to therm¬ ally shield the catalyst body from the homogeneous reac¬ tion zone. 2. The combustor of claim 1 wherein the catalyst body and the separator body together comprise a single, monolithic body. 3. The combustor of claim 1 wherein the catalyst body and the separator body comprise discrete bodies dis¬ posed in proximity to each other. 4. The combustor of claim 1 wherein the catalyst body and the separator body comprise discrete bodies dis¬ posed in abutting contact with each other. 5. The combustor of claim 3 wherein the catalyst body and the separator body are of substantially identical cross-sectional area and configuration. 6. The combustor of any one of claims 1, 2, 3 or 4 wherein the catalyst composition comprises a palladium ox¬ ide catalyst composition. 7. The combustor of claim 6 wherein the separator body is substantially free of a metal-containing catalyst effective for catalytically promoting thermal combustion of the inlet combustion gas mixture. 8. The combustor of claim 1 wherein the catalyst body comprises a plurality of catalyst members. 9. The combustor of claim 8 wherein each catalyst member abuts the catalyst member or members adjacent to it and the most downstream catalyst member abuts the separa¬ tor body. 10. The combustor of claim 9 wherein each catalyst member is in proximal relation to the catalyst member or members adjacent to it and the most downstream catalyst member is in proximal relation to the separator body. 11. The combustor of claim 8 wherein each of the ca¬ talyst members and the separator body contains from 9 to 400 of the gas flow channels per square inch of cross-sec¬ tional area ("cpsi"). 12. The combustor of claim 1 or claim 8 wherein the total combined length of catalyst members is from about 1/2 to 12 inches, and the length of the separator body is from about 1/2 to 5 inches. 13. The combustor of claim 1 or claim 8 wherein the separator body comprises at least one of (i) a silica-mag¬ nesia-alumina material comprised primarily of cordierite, mullite and corundum, and (ii) a ceramic fiber matrix ma¬ terial comprising ceramic fibers, the composition of which comprises alumina, boron oxide and silica, the fibers be¬ ing fixed in a silicon carbide matrix. 14. The combustor of claim 13 wherein the silica- magnesia-alumina material comprises about 20 to 40 weight percent Si0 2 , about 3 to 6 weight percent MgO and about 54 to 77 weight percent Al 2 0 3 , with from about 50 to 90 per¬ cent by weight of each of said Si0 2 , MgO and Al 2 0 3 com¬ prising crystalline material, the balance comprising amor¬ phous material. 15. The combustor of claim 14 wherein the crystal¬ line material comprises about 15 to 40 percent by weight cordierite, about 15 to 35 percent by weight corundum and about 10 to 30 percent by weight mullite by weight of the carrier. 16. The combustor of claim 15 wherein the fibers of the ceramic fiber matrix material comprises about 64 per¬ cent alumina, 14 percent B 2 0 3 and 24 percent Si0 2 . 17. The combustor of claim 1 wherein the catalyst body comprises a plurality of catalyst members and wherein the catalyst composition comprises palladium oxide dis¬ persed on a refractory inorganic oxide support. 18. The combustor of claim 17 wherein the refractory inorganic oxide support comprises alumina. 19. The combustor of claim 18 wherein the alumina support is impregnated with a rare earth oxide. 20. The combustor of claim 1 or claim 17 wherein the separator body has disposed thereon a coating of a materi¬ al that is substantially inactive for the combustion of the combustion gas mixture under ordinary separator body operating conditions. 21. The combustor of claim 20 wherein the coating on the separat :- body comprises alumina. 22. The combustor of claim 20 wherein the coating comprises a binary oxide of a rare earth metal and palladium that is substantially inactive at separator body normal operating temperatures.

Inventors: FEELEY JENNIFER S, FU JAMES C, LARKIN MATTHEW P, SIMONE DIANNE O, FEELEY, JENNIFER, S., FU, JAMES, C., LARKIN, MATTHEW, P., SIMONE, DIANNE, O.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1994
Main classification: F23C11
IPC codes: B01J23, F23C13, F23C99, F23R3